1. Field of the Invention
This invention relates to liquid crystal display systems and particularly to such systems as are operated so as to periodically reverse the direction of current flow into a liquid crystal cell thus prolonging cell operating life many times over.
2. Description of the Prior Art
Liquid crystal substances have been a scientific curiosity since their peculiar properties first began to be recognized. These substances flow, pour and assume the shape of their containers as if they were liquids. However, the molecules comprising these substances, unlike the molecules of liquids, tend to form loosely ordered arrays rather like the regular lattices of crystals. When these substances are excited by electric fields, turbulent activity may be set up within the substance. This turbulence may cause the material which was originally transparent, to become white because the turbulent fluid scatters light. This effect has been called dynamic scattering. The scattering effect can be halted and the clarity of the material restored simply by turning off the electric field.
Liquid crystal substances are currently being used to create new families of devices for display. A liquid crystal display device differs fundamentaly from an electronic display device such as a cathode ray tube which generates its own light. A liquid crystal device generates no light of its own, it scatters ambient light much as does this printed page. It has been noted that the liquid crystal is normally clear, but when an electric field is applied some regions in it become turbulent and scatter light. Images can thus be formed by controlling the shape and size of the turbulent regions caused by an incident electric field.
Since these liquid crystal devices reflect light instead of generating it, displays thus created can be viewed under a very wide range of lighting conditions including those which would wash out the image created on a light generating devices. Liquid crystal displays can thus be viewed in direct sunlight or in an ambient light field created, for example, by a flood light. Since liquid crystal devices do not emit light, they should require relatively little power and such is determined to be the case.
Liquid crystal display systems have been fabricated by sandwiching a thin layer of liquid crystal material between a glass plate which has been coated on one side with a conductive material forming a transparent electrode and a back plate having a matrix array of reflective electrodes formed on a semiconductor wafer.
Prior art devices have utilized a direct current in the application of the electric field. However, it has been shown that the operating life of liquid crystal materials can be increased many times by employing alternating current excitation rather than direct current excitation.
It would be of great benefit to the art to provide a display device which would dissipate much less power than the cathode ray tube and be usable in the same applications as the cathode ray tube. It would also be of great benefit to provide a display on which contrast could be maintained and even enhanced at high levels of ambient illumination. It would be of further greater value to provide a display exhibiting all the above advantages and in addition having a resolution uniform over the entire display area and whose resolution will be dramatically higher than that of a cathode ray tube.